This invention relates to a control circuit for semiconductor components which can be switched on and off and which form part of a converter circuit for converting direct current into three-phase current, which converter circuit is connected between a direct-current source and the stator of a synchronous machine having a cylindrical rotor, and which control circuit comprises rotor position sensors for sensing the position of the rotor with respect to the stator of the synchronous machine, and also a direct-current measuring device for measuring the value of the direct current.